In a mobile communication system including a plurality of cells, a mobile station UE (User Equipment) is configured to continue communication through cell switching when moving from one cell to another cell. The cell switching will be referred to as a “handover”.
In general, a mobile communication system is configured such that a mobile station UE moves to neighbouring cells, and in the mobile station UE, if received power of a signal from a neighbouring cell is larger than received power of a signal from a serving cell, then the mobile station UE is handed over to the neighbouring cell.
In addition, the received power of the signal from the neighbouring cell or the received power of the signal from the serving cell, for example, corresponds to received power (RSRP: Reference Signal Received Power) of a downlink reference signal which is transmitted from the neighbouring cell or the serving cell.
With reference to FIG. 7 and FIG. 8, an example of a handover procedure will be described in detail.
As illustrated in FIG. 7, in step S1, a mobile station UE measures received power of a signal from a neighbouring cell.
In step S2, the mobile station UE determines whether the received power of the signal from the neighbouring cell satisfies Equation 1 below.received power of signal from neighbouring cell+hysteresis>received power of signal from serving cell  [Equation 1]
When it is determined that Equation 1 is satisfied, the mobile station UE notifies a network of an event A3 for reporting the above-mentioned measurement result in step S2.
Specifically, as illustrated in FIG. 8, the mobile station UE measures received power of signals from a serving cell (a cell A) and a neighboring cell (a cell B) to be monitored, and determines whether to notify the above-mentioned measurement result using “hysteresis [dB]” and “TTT (Time To Trigger) [ms]” notified in advance.
That is, in FIG. 8, when the received power (radio quality) of the signal from the cell B continuously exceeds the received power (radio quality) of the signal from the cell A for a predetermined period “TTT” or more and over the “hysteresis”, the mobile station UE determines that the above-mentioned measurement result (Measurement Report) should be notified.
Here, it is assumed that a value calculated by the following Equation 2 and Equation 3 is used as the received power (radio quality) Fn of the signal. Specifically, in the mobile station UE, an upper layer is configured to perform a filtering process (L3 filtering) expressed by Equation 2 with respect to a measurement value by a physical layer.Fn=(1−a)·Fn−1+a·Mn  Equation 2a=½(k/4)  [Equation 3]
It is noted that a value of “k” in Equation 3 has been notified in advance from a radio base station to the mobile station UE, similarly to the “hysteresis” or the “TTT”. Furthermore, the “hysteresis” is a value provided in order to prevent a handover from a serving cell to a neighbouring cell from frequently occurring at a cell boundary, and may have a positive value or a negative value.
Furthermore, in step S3, if a notification of the event A3 is received, the network decides that the mobile station UE should be handed over to a cell according to the received event A3.
Next, when the network decides that the mobile station UE should be handed over, the network instructs information on the cell to be handed over with respect to the mobile station UE. Then, the mobile station UE is handed over to the cell and notifies the network of handover completion.
As described above, in the mobile communication system, when a radio base station eNB and the mobile station UE are in a connected state, handover control is applied.
Here, the mobile station UE compares the measured received power in the serving cell and the neighboring cell with a determination condition threshold value based on a parameter notified in advance, and determines whether to notify the network of the measurement result.
In other words, even just after the serving cell has been changed by the handover, when it is determined that the measurement result should be notified again, the mobile station UE notifies the network of the measurement result again.
This attributes to the fact that a parameter in the determination conditions is small. However, when the mobile station UE is located at the boundary between the serving cell and the neighboring cell, the handover control may be unnecessarily performed. Here, the parameter in the determination conditions, for example, indicates hysteresis or TTT.
Furthermore, when the parameter in the above-mentioned determination conditions is large, the above-mentioned unnecessary handover control may be reduced. However, specifically, at the time of high speed movement, the measurement result may not be notified at an appropriate timing, so that a handover may be delayed and thus appropriate control may not be performed.